A United Nations peacekeeper stands guard near the scene where about 200 civilians were reported killed during a recent attack in the town of Bentiu.

Reuters/Landov

South Sudan has been in a downward spiral for months, and now the United Nations says hundreds of civilians were rounded up and killed by a rebel group when it recently took control in the town of Bentiu, an oil hub.

The White House condemned the attacks as an "abomination" and implored the country's leaders to end the cycle of violence.

The U.S. was deeply involved in helping South Sudan establish independence from Sudan in 2011, which raises the question of what the U.S. is willing to do as the fledgling nation faces the growing possibility of a full-scale civil war.

And why have things gone so badly wrong in South Sudan?

When I arrived in the capital, Juba, at the end of last year, a shootout in a military barracks had sparked violent conflict around the country, pitting loyalists of President Salva Kiir against those allied with his political archrival, the former vice president, Riek Machar.

The two former revolutionaries, who both had experience in South Sudan's long war of independence against Sudan, both desired power. Each accused the other of undermining democracy in the world's youngest nation.

The international community, especially the U.N., insisted that political dialogue was the only solution to what still was being called a political dispute.

Peace talks were urged, and after long delays, undertaken in a luxury hotel in neighboring Ethiopia. A cease-fire was at last signed, and hailed as an important first step toward peace.

But the cease-fire did nothing to convince hundreds of thousands of South Sudanese that it was safe to return to their homes. Many took refuge at U.N. missions throughout the country, cramming into every available space, from the shade of buildings to satellite dishes.

Attempts by Indian and Nepalese peacekeeping soldiers to clear them from the grounds were steadily ignored. And conversations with those refugees suggested that something darker was brewing than mere politics. They told of mass graves as well as roundups and executions of rival ethnic groups. This, many feared, was a potential genocide in the making.

The cease-fire was no sooner signed than broken. Juba was quiet, but major cities proximate to the country's oil fields — Bor, Malakal, Bentiu — traded hands repeatedly, alternatively under government and rebel control.

The cease-fire had failed to solve the fundamental power struggle at the heart of this conflict: Which man would win power over South Sudan? President Kiir, the gruff, silent commander in his ubiquitous black cowboy hat; or Machar, the smooth-talking diplomat with a Ph.D. in philosophy?

Increasingly, this "political" conflict is being fought along ethnic lines.

On April 15, rebel forces loyal to Machar took over Bentiu, a northern town near the Sudanese border. They reportedly killed hundreds of Dinka civilians — men, women and children — taking refuge in a mosque. They also killed Sudanese traders from neighboring Darfur, Sudan. According to a U.N. report on the massacre, a local radio station was used by Machar's commanders to urge his ethnic Nuer supporters to commit acts of rape against women and girls from other ethnic groups.

Two days later, on April 17, gunmen wearing government army uniforms and youth militias professing loyalty to President Kiir stormed a U.N. compound with rocket-propelled grenades in the government-controlled city of Bor. Inside they opened fire on 5,000 Nuer civilians — mostly women and children — taking refuge. The death toll has not been determined.

For some observers it had echoes of the Rwandan genocide of 1994, when hate speech and poisonous ethno-political ideology was used to blur the line between "fighter" and "civilian." During those 100 "days of darkness," more than 800,000 Rwandans were killed.

The U.S. Role In South Sudan

It now feels like a long time since South Sudan became a nation. On that upbeat summer day, July 9, 2011, the Republic of South Sudan officially seceded from Sudan, ending a 50-year struggle marked by decades of civil war. In a referendum earlier that year, 99 percent of the population of southern Sudan voted for independence, and the celebrations on July 9 were euphoric and countrywide.

But from the beginning, it was a poor, fragile country that lacked many basic necessities such as infrastructure and functioning government institutions. American technical advisers filled the halls of a government propped up by billions of dollars in American aid. But soon there were reports of corruption and autocratic behavior by the Kiir government, who fired his entire Cabinet — including Machar — last year.

Machar slipped into hiding before he could be arrested by the Kiir government. The latest reports are that Machar's forces are "racing" to Bor to avenge the massacre there.

With no plans to put American troops on the ground in South Sudan, the Obama administration is relying on diplomatic pressure and threats of withdrawing aid. But the United States does not have the leverage it once had in South Sudan. The country has more than $1 billion in oil revenue that used to go to the north.

Pressure has been put on both Kiir and Machar to sit down and talk seriously. But neither man seems to be listening.

NPR correspondent Gregory Warner covers Africa from his base in Nairobi. You can follow him @radiogrego.